In order to correct the light quantity of a light source and variations in luminous intensity distribution, a conventional image forming apparatus such as a copying machine, scanner, or facsimile apparatus moves an original to a white reference plate for each original or at a predetermined timing, reads the original, and performs shading correction every reading. Alternatively, an end white plate is prepared outside the image region, and monitored to perform correction on the basis of the variation amount.
In the conventional method, movement to the white reference plate at a predetermined timing greatly decreases the reading efficiency of a contact image sensor (to be referred to as a CIS hereinafter). In the use of the end white plate, the light quantity is monitored at an end portion in the main scanning direction. Variations in luminous intensity distribution in the entire main scanning region cannot be corrected, readily degrading the image.